1. Field of the Invention
The present invention relates in general to the inspection of covered metal components and in particular to a new and useful method and device for the eddy current inspection of covered metal components using stationary magnetic fields and scanning sensor arrays.
2. Description of the Related Art
Corrosion on the exterior of components such as pipes, vessels and support structures is a pervasive problem throughout the petroleum and chemical process industry costing millions of dollars annually. A majority of these components are covered with material such as insulation which promote the corrosion by entrapment of water at the metal/cover interface. The removal of these covers and coatings for visual inspection is very costly and accounts for a substantial portion of the annual maintenance costs. Some methods have been developed in an effort to inspect covered components without removal of the insulation or covers.
One method developed for the inspection of pipes, tanks, and vessels through insulation is referred to as the Transient Electromagnetic Probe (TEMP). Two relevant patents have been issued; SPIES (U.S. Pat. No. 4,843,320) and Lara (U.S. Pat. No 4,843,319). This method uses the decay time of a diffusing eddy current pulse in the vessel wall to measure its thickness. The basic method is distinctly different from the low frequency eddy current (LOFEC) method in that transient decay time of diffusing eddy current is measured rather than flux field perturbations caused by a localized defect. Other distinguishing differences are:
1. TEMP measures the average wall thickness over a large (.gtoreq.16.sup.4 inch .gamma.diameter)--LOFEC detects the loss of surface material due to corrosion under insulation (CUI) in areas as small as 1" diameter.
2. TEMP is not a scanning technique--the very large probe head must be left in place for about 3 seconds to make a single measurement. The LOFEC method can be scanned at least as fast as 4-6"/sec (probably faster) continuously producing output signals. Therefore, the LOFEC technique can be used as an inspection, as opposed to a sampling tool.
3. There is no evidence that the TEMP method can handle the significant "artifacts" that produce signal perturbations in electromagnetic testing--these are aluminum cover overlaps, carbon steel retaining wires under the aluminum, circumferential weld beads, hidden taps or plugs, nearby support brackets, steam trace lines, etc. The LOFEC method has been designed to eliminate or minimize the effects of all those artifacts.
A second method which has been developed for the CUI problem is the portable, real-time x-ray system (LIXI.RTM.). Low energy x-rays are directed tangentially to the pipe so that they penetrate the insulation but not the pipe wall, thus imaging the corrosion area. This technique is much too slow to be used as an inspection tool to cover long lengths of pipe. The slow speed is due to a very limited field of view and the many tangential shots required to look at just one axial location on the pipe. It would be best suited to do spot checks for confirmation of corrosion damage after detection by a scanning method such as LOFEC. A second serious problem with the portable x-ray method is that scale in the corrosion site may tend to hide the corrosion damage.
However, these methods have major drawbacks. Particularly, ultrasonic inspection methods are severely limited by the need for fluid couplant, scattering of the ultrasound within the cover, air gap attenuation, mechanical alignment problems, and the poor resolution resulting from the relatively low operating frequencies required to penetrate the cover. Microwave techniques require removal of aluminum weather barriers, are adversely affected by moisture, and are scattered by insulation covers and by the shape of the corroded areas. X-radiation methods are time consuming, hazardous, relatively expensive, and are limited by the size and accessibility of the covered components.
In response to the deficiencies found in the methods listed above, the low frequency eddy current (LOFEC) method was developed for detecting corrosion and other defects on the surfaces of metal components that are covered with various materials such as paint, foam rubber, marine growth, calcium silicate insulation and relatively thin metal sheets. The object of the LOFEC method is to detect surface defects such as corrosion on the component while leaving the covering material intact.
FIGS. 1-5 illustrate a basic LOFEC probe generally designated 2 used for detection of surface defects such as corrosion under covers. The LOFEC probe depicted in FIGS. 1-5 comprises an inverted U-shaped yoke 10 having legs 11 placed on a cover 40 of a component 44 such as a steel plate. An excitation coil 20 is wound about the magnetizing yoke 10 between the legs 11. An alternating current 4 composed of one or more sinusoidal components is generated and applied to terminal 22 of the excitation coil 20. This alternating current 4 produces an alternating magnetic field 18 in the inverted U-shaped yoke 10. The yoke 10 guides the magnetic field through the cover 40 and into the component 44 beneath. If the component 44 is a ferromagnetic steel, the magnetic field 18 will be concentrated in the plate and directed from one leg of the yoke 10 toward the other. The alternating field 18 induces eddy currents 8 in the steel and other metals, (e.g., aluminum covers 40), located between the probe and the steel. The induced currents 8 tend to flow between and around the legs 11 of the U-shaped yoke 10 as illustrated in FIG. 1. Both the current 8 and the magnetic flux 18 are concentrated in the materials near and under the yoke 10.
FIG. 3 shows that a magnetic flux sensor 30 is located between the legs 11 of the U-shaped magnetizing yoke 10 beneath the excitation coil 20. The sensor 30 lies in a plane passing through the cross-section of the legs 11. The flux sensor 30 is an electronic device, such as a coil of conducting wire or a Hall element semiconductor that provides a signal response voltage proportional to the intensity of the magnetic flux 18 intercepted by the sensor 30. Under normal conditions, e.g., a uniform steel structure with no surface defects, the magnetic flux 18 and induced eddy currents 8 in the region directly under the excitation coil windings 20 are parallel to the plane formed by the sensor 30 that intersects the legs 11. The magnetic flux 18 flows from one leg 11 to the other and induced current 8 flows perpendicular to the flux 18. The presence of a near surface defect 55 in the steel component 44 such as corrosion causes a change in the magnitude, phase and direction of the induced currents 8 and associated magnetic field 18 within the steel 44 and in the region between the steel 44 and the probe 2.
Surface defects 55 are identified by scanning the probe 2 over the cover 40 of the structure 44 and detecting the signal response voltage, observed at terminals 33 of the flux sensor 30.
However, presently there is a need for a method or device for reducing or minimizing extraneous and unwanted signal responses caused by variations in the geometry and electromagnetic properties of the component 44 when using the LOFEC technique.
Additionally, there is a need for a method or device which can detect defects on metal components covered with marine growth while minimizing extraneous signal responses.